1. Field of the Invention
The present invention relates to an electroless copper plating solution and a process for electrolessly plating copper. More specifically, the present invention relates to electroless copper plating solution and process for forming a copper plating layer used as a conductor of a printed-circuit board or a ceramic substrate, a shielding material for electromagnetic waves, or the like.
2. Description of the Related Art
Electroless or chemical copper plating baths using ethylenediaminetetraacetic acid (EDTA) or Rochelle salt (i.e. potassium sodium tartrate) as a complexing or chelating agent for copper ion are well known. Particularly such baths using copper sulfate as a copper salt and formaldehyde as a reducing agent are widely used. These electroless copper plating solutions are, however, disadvantageous because of low deposition rates, etc. Recently, to reduce the cost of, for example, printed boards, a demand has arisen for electroless plating solutions having a faster deposition rate. To meet demand, electroless plating baths using an accelerator or an activating agent with a reducing agent have been proposed, but these baths have proved unsatisfactory and it has become necessary for even faster plating baths to be developed.
Japanese Unexamined Patent Publication (Kokai) No. 60-70183 corresponding to U.S. Pat. No. 4,650,691 discloses an electroless copper plating process stabilized by adding a metal cyano complex as a stabilizer and a complexing agent for the metal of the metal cyano complex in an electroless copper plating bath, wherein an alkanolamine is used as the complexing agent for the metal of the metal cyano complex. However, in this process, another complexing agent for copper ion is included in the bath and the function of the alkanolamine as an accelerator is not effected.
Japanese Unexamined Patent Publication (Kokai) No. 59-143058 discloses an electroless copper plating solution containing triethanolamine, which provides a high plating efficiency even if the solution is prepared with inexpensive chemicals. However, a separate complexing agent for copper ion is included in this solution, further the description does not state that triethanolamine acts as an accelerator, but states that an addition of triethanolamine in an amount of more than 0.01-0.5 g/l causes decomposition of the plating solution and reduces the plating efficiency.
Japanese Unexamined Patent Publication (Kokai) Nos. 60-218479 and 60-218480 discloses an electroless copper plating solution giving a copper plating layer having excellent properties, in which an alkali-soluble inorganic silicon compound and an inorganic or organic compound for stabilizing the plating bath are included. These publications also disclose that, as a complexing agent for copper ion, chemicals having a structure with a &gt;N--C--C--N&lt; skeleton are preferable, and the use of triethanolamine causes problems.
Trialkanolamines are mentioned in several publications as examples of the complexing agent for copper ion in an electroless copper plating solution (for example, U.S. Pat. No. 4,301,196, Japanese Unexamined Patent Publication (Kokai) Nos. 59-25965 and 60-245783 the latter corresponding EP-164580). However, these publications disclose only that a trialkanolamine can be generally used as a complexing agent for copper ion, and do not teach or suggest the effectiveness of the trialkanolamine used in an excess amount as not only a complexing agent but also an accelerator, and further, do not disclose any experimental data in which a trialkanolamine is actually used as a complexing agent for copper ion in a plating bath.
"Electroless Plating and Plating of Plastics" (Plating Technology Materials (2), Japan, 1973) includes description of a little-known Russian academic report of an experiment wherein trialkanolamine was used as a complexing agent for copper ion in an electroless copper plating bath. However, a deposition rate of only about 1.5 .mu.m/hr was reported in that paper.